Open Source Unicycle? - Designs and technical discussion

Following the announcement that the BrakeFast adaptor is going to be an Open Source Hardware product, and most unicycles are using the free ISIS crank interface standard, it got me thinking about whether there’d be any merit to having fully Open Source unicycle component designs.

Companies already often work together on elements of designs or products, or just use parts produced by another company but specifically designed for unicycles:

  • Qu-Ax uses the KH design Eleven saddle
  • The KH36 used to use a variation of the Nimbus Stealth2 rim with braking surface, now they use a standard Stealth 2 and in Europe a Qu-Ax King George tyre
  • Florian Schlumpf worked with KH for the MUni Schlumpf hub

People have made freely available bicycle designs/components, why not unicycle components?

So, as some design practise, I’ve been making some parts (bearings, spacers, some hub parts, and a 36" Stealth 2 inspired rim so far).

I know people have created their own unicycle frames, freewheel hubs, geared hubs, 3D printed cranks and other accessories - how many of these are open designs? Would it be in any way useful to have these collated in one place for others to iterate on or work from in the future.

If anyone’s created something and would be happy for it to be an open design, but didn’t go any further than a paper napkin drawing, I’ll also put myself out there as being happy to recreate the part as a 3D model in Solidworks and/or FreeCAD.


I’m happy to share my Corbin V-36 design.

I have it in SketchUp somewhere…


Man the ISIS interface is a bit of a head scratcher to actually model.

Finally got there and my crank interface correctly meshes with my axle interface, complete with the correct sample pre-load spacing as defined in the specification so I’m calling it a success.

Now I just need to see if I can do it in FreeCAD!

Well I think the Brakefast idea came from my idea and subsequint making of the first disc adapter for the original Schlumpf hub and then the more recent versions that I gave away to my friends who had those hubs and to those who asked when I had them. But the best thing is how Roger and team put their heads together and made many improvements to the design/idea and now all my unicycle friends can have them. I had no intention of ever producing them since it was only, and still is a hobby of mine to make what I want. They are taking all the chances of producing a great product, and the financial hit. PROPS to them and thank you from the in frame schlumpf disc community! (Braking not through the gears)


Thanks for sharing, unicycle community! Imagine what would happen if unicycling were universally popular and if the Brakefast adapter were something everyone needed. The Bill Gates Foundation would have to intervene to make sure the adapter was 100% proprietary.


Just got invited to this website by a user on the Unicycling Discord. Here is my entry, a replication of the part that Ed Prat used to mount his perpendicular tubes for his pannier bags. Unfortunately I cant upload the Fusion 360 file to this thread, but I can show an image of it. Just contact me if you want the file.


I can share the stuff I have if I can find it.


We are going to release the design fully, but we are trying to be sure that it is documented and we can show some version control. Please give us a few weeks, it may seam like a slow process, but we are getting there.


So I was looking at the ISIS spec again earlier and I wondered whether anyone had defined the spindle (axle) designation that’s typically used on unicycles?

The ISIS spec includes:

Am I correct in thinking that the “normal” unicycle designation would therefore be 126 for a 158mm wide axle?

110mm bearings centre to centre, an ideal 7mm spacer on either side (to give us 126 as DIM ‘B’), then 16mm of ISIS interface on either side to give 158mm width.

By the same logic then 125mm hubs would be a designation of 151, and 90mm hubs would be a designation of 116, although I can't find the length of any hub axles to check if the logic holds up.

Then the latest generation Schlumpf hub would be at the low end of a 124 designation with 16.4mm of ISIS interface on either side:

This larger 16.4mm for the interface makes sense to ensure that the cranks are installed tight enough to handle the insane forces put on them when you’re using a crank mounted disk brake.

Does this all seem logical?

It should also be possible to quantify hubs based on their dimensions at the defined positions from the spec on the interface to give each a “real world” spindle/axle designation based on the exact interface positions.

I’m not sure how useful that would be though as spacers are best chosen based on real world crank installation to account for manufacturing variance and interface wear.

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I found an unknown 92mm hub (possibly an Exceed Ti or something based on the tiny flanges?), which by my previous calculations would be a 118 (with 7mm spacers), giving 15.5mm of ISIS interface.

If it is a Ti hub, perhaps that’s realistic (but it would be out of the ISIS interface spec), or perhaps it was designed for 6-6.5mm spacers.

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112mm outside to outside, but yes, the rest sounds logical.

I’m just not sure what use that designation exactly that is, since it’s not specified in the standard. If you “misuse” a standard like this for a different product like unicycles, you would give your own dimensions to a manufacturer anyway - extrapolating the designation only leads to potential error.

And if you only want a quantification of how far the cranks are apart to then be able to calculate “Q-factor” (in the original sense, so how far the pedals are apart axially) for different cranks, the easiest measurment to use will be “DIM B”. (Outside distance of the “specified” or actually useful spacers).

So the 112mm outside to outside of the bearings wouldn’t include the spacers so it’s not a useful measurement, and clearly hubs are made with a specific size spacer in mind for the ISIS interface hard stop (at ~16mm).
Hubs that don’t use 12mm wide bearings (Impact Pro models), should/would still have the same designation even with different width bearings.

That “DIM B” is the spindle designation with acceptable tolerances.

I suppose it just feels to me like hubs are made to a spec, but that spec isn’t generally provided and it should be. Are all 100mm hubs made to a 126mm design (clearly the Schlumpf isn’t for a start), or are some designed to be used with smaller spacers for some reason? I know that not all of the 100mm hubs I have are quite the same and I presume that’s down to design rather than rubbish manufacturing tolerances.

In theory it should enable people to buy the correct spacers to go with their hubs without having to have both the hub and cranks in their hands to actually test.
It would enable someone to buy a replacement hub knowing that the interface is in the same place.

If all 100mm hubs are not made to the same DIM ‘B’ dimensions then why not? Is it because everyone starts from scratch with their interpretation of the unicycle usage of the ISIS standard? If so, that’s exactly the kind of reason why I started this thread.

If hubs were specified with a spindle/axle designation, then cranks could be sold with a + or - figure should they have deviated from the spec, or wish to have a different load on the interface (for example perhaps the Spirit cranks with their disk brake might want a higher interface loading - perhaps a -0.5 or -1 designation).

KH Moments could be sold with a +1 designation (your spacers want to be 1mm longer than standard) - UDC UK lists them as wanting 8mm spacers
Nimbus Ventures could be sold with a -1 designation (your spacers want to be 1mm shorter than standard) - UDC UK lists them as wanting 6mm spacers.

Yes, that is a TI hub. I recognise my drawing.

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Anything you can share on the width/spacer/interface position situation as someone who’s designed numerous hubs for manufacture?

Am I talking complete rubbish?

I will dig out my notes sometime. The problem is that the tolerances is quite high in the standard and I cannot remember what we decided as a compromise to get things to work in reality. :slight_smile:


Why you dont want to use Qu-Ax/Hollowtech style splines as opensource standard?

Because it’s not a freely available standard?

I also remain unconvinced that it’s necessarily the best way forwards, but I do own a unicycle with it, so we’ll see.

I also don’t think we want to have “a standard”, rather just open source designs of parts wherever possible to speed up development and enable people/manufacturers to easily keep things consistent.

Because it’s not a freely available standard?

I feel there isn’t any patent or license standard for qu-ax hub. Anyway splines standard is open.

So from my previous patent searches, I believe that Shimano have a variety of patents that cover their Hollowtech II setup (and a trademark on the name), but they don’t seem to apply to how the design is being used by Qu-Ax.

If anyone wants to model and verify the Q-Axle interface then that’d be great, but unlike the ISIS interface there’s no information out there on dimensional requirements (as far as I know), so it’d be quite the task.